Radioactive tracer system to indicate drill bit wear or failure

ABSTRACT

A radioactive tracer system for indicating drill bit wear or failure utilizing radioactive krypton 85 in clathrate form, in the form of water-soluble kryptonates, or dissolved in grease. Preferably the radioactive krypton is placed so that when drill bit wear or failure occurs, the radioactive krypton 85 is released and effectively becomes diffused in the circulating drilling fluid. At the surface, the radioactive krypton 85 gas is separated from the circulating drilling fluid by gas-mud separating means and is transported as a gas to a counting chamber where an accurate radioactivity count of beta rays released from the krypton is obtained. The beta rays indicate drill bit wear or failure when detected above the normal background radioactivity of the recirculating drilling fluid. The radioactive krypton may also be implanted in the teeth or other surfaces of the cones of drill bits or implanted in the shirttail or other body parts of the bit.

United States Patent [191 Fries RADIOACTIVE TRACER SYSTEM TO INDICATE DRILL BIT WEAR OR FAILURE [75] Inventor: Bernard A. Fries, Orinda, Calif.

[73] Assignee: Chevron Research Company, San Francisco, Calif.

[22] Filed: Aug. 18, 1972 [21] Appl. No.: 281,869

Related US. Application Data [63] Continuation-impart of Ser. No. 55,635, July 17,

1970, abandoned,

[52] US. Cl. 250/303, 250/304 [51] Int. Cl. G2lh 5/02 [58] Field of Search 250/303, 304

[5 6] References Cited UNITED STATES PATENTS 2,658,284 11/1953 Arps 250/303 X 2,957,986 10/1960 Quigg 250/303 3,299,269 1/1967 Hanson et al 250/304 1 June 18, 1974 Primary Exar niner-Archie R. Borchelt Attorney, Agent, or Firm-R. L. Freeland, Jr.; E. J. Keeling; J. A. Buchanan, Jr.

[ ABSTRACT A radioactive tracer system for indicating drill bit wear or failure utilizing radioactive krypton 85 in clathrate form, in the form of water-soluble kryptonates, or dissolved in grease. Preferably the radioactive krypton is placed so that when drill bit wear or failure occurs, the radioactive krypton 85 is released and effectively becomes diffused in the circulating drilling fluid. At the surface, the radioactive krypton 85 gas is separated from the circulating drilling fluid by gasmud separating means and is transported as a gas to a counting chamber where an accurate radioactivity count of beta rays released from the krypton is obtained. The beta rays indicate drill bit wear or failure when detected above the normal background radioactivity of the recirculating drilling fluid. The radioactive krypton may also be implanted in the teeth or other surfaces of the cones of drill bits or implanted in the shirttail or other body parts of the bit.

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RADIOACTIVE TRACER SYSTEM TO INDICATE DRILL BIT WEAR OR FAILURE This application is a continuation-in-part U.S. Pat. application of Ser. No. 55,635, filed July 17, 1970, now abandoned.

My invention is related to the detection of drill bit wear and failure in a downhole drilling environment. It specifically encompasses the use of radioactive krypton 85 gas, packaged in a manageable form, which is placed in selective component parts of a drill bit in combination with a simple and sensitive gas detection system at the surface capable of monitoring the presence of radioactive gas in the drilling fluid flow.

Drillers and their backup researchers in the oil industry have long been confronted with the problem of devising a precise means of determining when a given drill bit fails to function properly or efficiently. Gross drill bit failure is eventually detected by a marked drop-off in the rate of drill bit penetration. Minor decreases in the rate of penetration are generally not a positive indicator of drill bit wear or failure because such decreases can frequently be attributed to encountering hard rock formations. On the other hand, a high rate of penetration is not necessarily a positive indicator that a given drill bit is functioning properly because softer formations are also encountered. Since there was no accurate way of knowing what type of formation had been encountered without taking core samples or employing well-logging techniques, both of which are highly impracticable in the ordinary drilling situation, the rate of penetration is usually not a positive indication of the physical condition of the drill bit. The only means, then, that field drilling engineers rely on to detect drill bit failure are complete drill stoppage, the detection of drill bit debris in the effluent mud flow, a knowledge of how long a given drill bit had been in use as compared to the average expected lifetime of the drill bit, and vibrations in the drill string which sometimes occur if a drill bit is not functioning properly.

The concept of using a radioactive substance as a tracer in a drilling fluid or mud environment is disclosed by J. B. Warren in U.S. Pat. No. 2,468,905; it is also shown .I. J. Arps in U.S. Pat. Nos. 2,658,725 and 2,659,046. Warren proposes that a compressed gas means to be used to facilitate the mixing of a radioactive tracer with the drilling mud when a predetermined level of wear had been reached. Arps discloses that the radioactive tracer should be selectively released downhole in order to monitor formation characteristics in lieu of pulling the drill string and employing conventional logging techniques. J. A. Rickard, in U.S. Pat. No. 3,115,576, discloses the use of a radioactive tracer in a drilling mud to determine whether a balanced circulation is maintained. T. J. Nowak, in U.S. Pat. No. 2,692,755, proposes the continuous logging of the natural radioactivity of drilling mud due to its contact with the downhole formations and to the inclusion of drilling debris in the mud. In U.S. Pat. No. 2,658,724, noted above, J. J. Arps proposes that radioactive tracers be utilized to indicate the existence of abnormally high temperatures downhole to detect undesirable drilling conditions. He discloses the use of an ionization chamber immersed in the mud flow to detect the presence of radioactive tracers within the mud. He also discloses a number of radioactive salts and the possible use of a radioactive gas under pressure, but does not discuss the problems to which the present invention is addressed.

The combination of a radioactive substance in a vial enclosed in a drill bit and a cutter which ruptures the vial when the axis of the cone ceases to coincide with the axis of the drill shaft was disclosed by J. W. Graham in U.S. Pat. No. 3,011,566.

British Pat. No. 1,126,916 (Farbwerke Hoehst) discloses the use of radioactive krypton in a clathrate form that is sealed in a capsule in a drill bit. An expellant or propellant, to force the krypton 85 clathrate into the drilling fluid is specified to assure inclusion in the drilling fluid returns when wear causes rupture of the capsule. However, no provision is made for r'emoving the tracer from the drilling fluid. This has two primary drawbacks. One is that the tracer recirculates with the drilling fluid so that simple decay or evaporation from the drilling mud pit are the only ways of reducing radioactive background in the drilling fluid. This makes subsequent detection of other tracer peaks (from other drill bit failures) more difficult. The other drawback is that the patentees propose to detect the radioactive krypton by immersing a radiation detector in the drilling fluid. Unfortunately, radiation emitted by radioactive krypton 85 is 99.6 percent beta rays and only 0.4 percent gamma rays. Beta rays are readily absorbed by drilling fluid so that unless the krypton 85 is removed from the drilling fluid, the sensitivity of the detection is inordinately poor as compared to measuring the radiation in gaseous form.

All such previous proposals failed commercially. In some, no suitable radioactive isotope was used. The half-life was either too short so that difficult problems of manufacture and logistics were created, or the isotope was in a form that left a residue which contaminated the environment after they were used. In some a solid radioactive substance was carried to the surface in the drilling mud flow but was not mixed with the drilling mud flow. In a fast-flowing mud stream, it was possible that such a spurt of radioactivity associated with the solid radioactive substance could be missed by even a relatively sensitive counter. None of the proposed systems had the feature of separating a beta ray emmitting radioactive material as a gas from the mud flow prior to counting the level of radioactivity.

It is one object of my invention to implement a detection system which measures the activity level of the radioactive tracer gas after the tracer gas has been removed from the circulating drilling mud so that absorption of the tracer radiation by the drilling mud is eliminated.

A further object of my invention is the utilization of a detection system which counts radioactivity after most of the radioactive tracer from a failed drill bit has been removed from the drilling mud flow so that the background level of radioactivity in the drilling mud is not significantly built up over a period of time by subsequent periodic releases of tracer by other drill bit failures.

A more complete understanding of the elements and operation of my invention may be had by reference to the drawings which are hereby incorporated in this specification and in which:

FIG. 1 is a schematic illustration of the overall operation of my invention, illustrating an embodiment of the gas-mud separation means.

FIG. 2 is a cross-sectional view of a typical sealed bearing drill bit, illustrating the relation between the drill bit, the drill cone, the grease reservoir, the ring bearing, the ball bearings, the teeth, and the ball bearing race, and also illustrating the locations at which radioactive krypton 85 can be placed whether the krypton is a gas dissolved in the grease, in solid clathrate form, or in water-soluble kryptonate form.

FIG. 3 is a strip chart record of the amount of radioactivity measured at the surface when krypton 85 in dissolved form was introduced into the drilling mud flowing back to the surface at time zero. The initial peak is sharp and represents the detection of the extracted krypton 85 in gaseous form upon completion of the first cycle of the circulating drilling mud. By such physical extraction, the second peak is one-tenth as intense and is spread out over a longer period of time. The residual recycled radioactive krypton 85 is more widely dispersed throughout the circulating drilling mud and is of such a low intensity that it does not interfere with subsequent runs.

In essence, my invention is based upon the sensitivity and accuracy with which it is possible to measure the radioactivity of a separated sample of a gas and upon the fact that krypton 85 gas has the property that it can be packaged in a non-gaseous form which readily dissolves or disperses upon release into the circulating drilling mud. Krypton 85 gas is suitable because its halflife is years and therefore it can be placed in drill bits at some convenient time prior to use and does not necessarily have to be implanted in the drill bit at the drilling site; any gas with a half-life greater than about 5 days possesses this desirable property. Once krypton 85 has been prepared in a clathrate from, has been kryptonated onto a water-soluble salt or physically dissolved in bearing grease, it can be easily placed in a drill bit. When drill bit wear or failure occurs, such a dissolved form of krypton 85 comes into contact with the circulating drilling mud and, in each case, the krypton 85 quickly becomes diffused in the drilling mud. The water-soluble salts dissolve in the drilling mud, the clathrate decomposes upon contact with the mud and the liquified grease flows with the mud. Thus, after the mud flows from the borehole, a gas-mud separator and a gas detection system is capable of accurately determining how much radioactive gas has been released below. Krypton possesses the desirable property that it will not react with the constituents of the circulating drilling mud and, therefore, reaches the surface intact other noble gases also posses this property.

Clathrates are a newly discovered group of inclusiontype compounds in which molecules of a captive substance are enclosed within the crystalline structure of a host substance. Clathrates of radioactive krypton 85, as disclosed in Radio-Release in Review with Special Emphasis on Krypton 85 Clathrates and Kryptonates, Carden, Joan E., Oak Ridge National Laboratory Publication No. ORNL-llC-IS, can be prepared from an aqueous solution or from a melt. For example, a 0.5 to 1.0 gram sample of solid hydroquinone is put in a pressure bomb, a vacuum is achieved to eliminate atmosphereic gases and the bomb is pressurized with krypton gas containing a small percentage, say 5 percent, of radioactive krypton 85. The bomb is maintained at 185 C. for approximately 2 hours to allow the krypton to saturate the melt. Then the system is cooled to room temperature over a period of several days. Finally, the fused krypton 85 hydroquinone clathrate is removed and utilized in a powdered, granulated or pelleted form.

Clathrates of radioactive krypton can be used in a variety of ways to detect drill bit wear and failure. The clathrate can be placed directly in the grease reservoir of a sealed bearing drill bit. Or, the clathrate can be placed within or underneath the teeth in the cones of a drill bit, within holes drilled in the surface of cones of drill bits and in the sides of the shirttail of drills. It has been found that only very small amounts of the clathrate need to be used. For example, in one series of experiments a few milligrams of a hydroquinone clathrate of krypton 85 was mixed with several hundred milligrams of powdered sugar. A small amount of stearic acid was added as lubricant. Then, the mixture was placed in a pelletizer and tiny pellets were prepared. These pellets were placed in the aforementioned locations in a drill bit and the drill bit was used to drill an exploratory hole.

Kryptonates are prepared by either ionizing krypton atoms and impelling them into a solid or by placing krypton gas and a solid in a pressure bomb under high pressure so that the krypton atoms diffuse into the solid. Over a hundred solids have been found capable of accepting krypton atoms in that manner but it is found that solids which contain regular structural voids on an atomic scale are preferable. Boron nitride, red iron oxide and platinum dioxide have been found to be suitable for oil field use. Ordinarily, the krypton held captive in kryptonates is released only by heating. However, water-soluble kryptonates release the captive krypton when the solid comes in contact with water. Therefore, water-soluble kryptonates can serve as model tracers because they release their krypton as soon as they come in contact with the drilling mud downhole.

Radioactive krypton may also be incorporated in grease without first being compounded as a clathrate or as a kryptonate by the following simple procedure. A conventional lubricating grease such as that used in automobiles and trucks chassis or wheel bearing lubrication is placed in a rubber bag, or balloon, and a bubble of radioactive krypton 85 gas introduced. The grease is then kneaded until the gas becomes dissolved. The gas is soluble in the grease and effectively disperses without leaving residual bubbles. The grease could also be prepared during mixing of the original grease batch by enclosing the chamber under pressure and stirring the grease during compounding with a quantity of the radioactive krypton gas in the chamber. The material is stable even at relatively high temperatures but upon exceeding a particular temperature, say 250 F., would liquify and escape into the drilling fluid. This would happen, for example, when a bearing fails in a drill bit so that the cones will not turn, direct friction between the bearing supported cones and the rock being drilled will cause the grease reservoir temperature to rise sufficiently so that the grease is liquified and runs out of the bearing support. Such failure is relatively rapid after the melting point of the grease is reached so that the melted grease containing the radioactive krypton is readily incorporated in the drilling fluid and returns to the surface as a part of the drilling mud returns. Another mechanism of grease discharge is also possible; intact grease may spurt out of the bearing seals without bit heating, upon mechanical failure of the bearing seals.

A particular advantage of using krypton 85 dissolved in grease is the low cost of krypton tracer in this form.

The cost comparison being on the order of two cents per millicurie versus about ten dollars per millicurie for krypton in clathrate or kryptonate form.

In FIG. 1 drill bit 13 with drilling cones 14 is being worked to increase the depth of wellbore 12. Drilling debris resulting from the extension of wellbore 12 into formation 15 is carried to the surface by the upward movement of drilling mud ll. Upward flowing drilling mud 11 has been introduced at the bottom of wellbore 12 by an opening in the bottom of drill bit 13 after flowing from the surface to drill bit 13 within annular drill string 10. Clathrates and kryptonates of radioactive krypton 85 or physically dissolved krypton 85 released at the bottom of wellbore 12 due to wear, bearing or tooth failure, is caught up by upward flowing circulating drilling mud 11 and transported to the surface.

At the surface in this embodiment of my invention upward flowing circulating drilling mud 11 is channelled through conduit 18 and discharged as flow 19 into separation trough 20. The drilling mud moves by gravity flow through separation trough 20 and is discharged as flow 24 into automatic shaker screen 28. As it passes through separation trough 20 the circulating drilling mud is agitated by stirring means 21 comprising electric motor 22 and stirring blade 23. The lower end of stirring means 21 is immersed in the drilling mud flow so that stirring blade 23 is constantly agitating the mud and so that outer skirts 9 maintain an air-tight seal with the flowing drilling mud. Fresh air is drawn into stirring means 21 at inlet 8 by means of the negative pressure within stirring means 21 created by vacuum pump 31 and communicated to stirring means 21 by conduit 30. As krypton gas containing radioactive krypton 85 is separated from the flowing drilling mud by the agitation of stirring blade 23 it is caught up in the flow of fresh air from inlet 8 and carried through conduit to the counting means 36.

Beta and gramma ray counting means 36 contains a counting element which can be a Geiger tube, a scintillation counter, an ionization chamber, a proportional counting means or any other beta and gamma ray counting means which is reasonably compact and inexpensiv'eand therefore suitable for field use. Counting element 35 is calibrated under laboratory conditions to determine-its response as a function of the amount of radioactivity, the rate of air flow, and the size of the chamber component of counting means 36. The output of counting element 35 is monitored continuously by rate meter 38 equipped with an instantaneous readout dial 7. Instantaneous readout dial 7 may be equipped with a visual or audible alarm means which is activated whenever the instantaneous intensity of radioactivity exceeds a predetermined level. Finally, a record 41 of the level of radioactivity as a function of time is kept on continuous strip chart 39 by means of galvanometer pin 40 driven by count rate meter 38.

After passage through the chamber of counting means 36, air containing radioactive krypton 85 and non-radioactive krypton is transported by conduit 37 to vacuum pump 31 and thence into the atmosphere Both forms of krypton in the atmosphere, do not become concentrated in the region in which drilling is conducted. Streams or. ground water are not in danger of being polluted. Animal life such as cow 42 andplant life such as alfalfa 6 are also not in danger of taking in concentrated doses of radioactive krypton. Exposure of the drilling crew and engineers to direct radiation, i.e.,

to beta and gamma rays, is minimal because of the low activities of the samples used, the rapid diffusion into the atmosphere and the absorption of the drilling mud as well as the absorption of other components of the separating and counting means.

After circulating drilling mud 24 flows from separating trough 20 into automatic shaker screen 28, a major portion of the radioactive krypton has been removed. The residual radioactivity is not great enough to obscure any readings that are taken after a given portion of the mud completes another cycle through drill string 10 and out conduit 18. Automatic shaker screen 28 serves to recover formation cuttings in the drilling mud and separates some of the drilling debris from the drilling mud. Cleaner and more uniform drilling mud is then stored in open pit 29 to be recirculated downhole. When drilling is underway, pump 49 draws drilling mud from open pit 29 through conduit 50 and forces it thence through flexible hose 47, swivel head 46, drill pipe 48, and drilling head 17 to drill string 10. A relatively uniform flow is maintained by the pressure imparted by pump 49 to the drilling mud introduced to drill string 10.

Referring now to FIG. 2, a cross-sectional view of a single cone of a three-cone sealed bearing drill bit, it can be seen that teeth 87 are implanted in a predetermined pattern on the surface of cone 89. Cone 89 is disposed to rotate about the lower end of shank 82 by means of sealed bearing 78 and ball bearings 90, and thereby comes in contact with the formation to be drilled. Ball bearings and sealed bearing 78 move freely in race 72 by means of grease which is introduced to race 72 from grease reservoir 70 via channel 71. Ring seals'80 are constructed of-a suitable material such as spring steel and serve to prevent grease from escaping from race 72. Grease reservoir 70 is typically constructed as shown with cylindrical body-83 having a rubber boot 76 at one end to maintain the grease in the reservoir under slight differential pressure and having holes 79 opposite race 95, said race being in communication with channel 71, and said holes 79 permitting grease to be supplied to the ball and sealed bearing race 72 via channel 71 whenever necessary. Cylindrical shell'83 of grease reservoir 70 is maintained in tight communication with shank 82 by means of 9 rungs 92 and 93. Grease is loaded into reservoir 70 at the factory through the threaded bore opening filled by set screw 81. A grease nipple (not shown) is usually threaded into bore 100 for filling with a conventional grease gum. This can also be done at the drill site by temporary removal of set screw 81 and substitution of a grease nipple. The nipple is then replaced by set screw 81. In this manner, small grease wads containing dissolved radioactive krypton 85 can be injected into the bits grease reservoir 70 just before the bitis run into the well on the end of drill string 10. Such a procedure gives indefinite shelf life to thebit since the tracer is added at the time of use only. Additionally, only minor quantities of radioactivegrease is required so that cost per bit is greatly reduced.

External pressure on rubberboot 76 is maintained by the hydrostatic pressure of the drilling mud. This pressure is communicated to rubber boot 76 by drilling mud 84 which has passed through screens 77 and 91 and is contiguous with the external drilling mud. There is equal pressure on both sides of rubber boot 76. Only a very slight differential pressure develops which forces the grease out of the reservoir as needed.

As indicated above, a wad or small quantity of the radioactive'krypton 85 activated grease is desirably inserted near the top of reservoir 70 test. Tests indicate that even under downhole temperature conditions the radioactive grease does not diffuse into the: regular body of the grease until such time as some portion of the drill bit fails, as for example by damage to the cones, the bearing supports, or the bearings themselves. However, excessive wear of the drill cone 89, sealed bearing 78 or other parts of the rotating bit including the shirttail 97, raises the drilling temperature due to increased transfer of heat of grinding rock so that the grease melts. The direct transfer of heat between the grinding elements such as teeth 87 or the body portion 89 will heat reservoir 70 sufficiently so that the grease is liquifled and a plug of grease, such as that indicated by phantom location 75, will run out of the resevoir and drill bit bearing. The liquified grease containing the radioactive krypton is then picked up by the drilling fluid returns and carried to the detecting system at the earths surface. Alternatively, mechanical failure of the bearings can also release the grease without any significant temperature rise of the bit.

Clathrates and kryptonates of radioactive krypton 85 may be placed as shown at various locations throughout the drill bit. In one embodiment of my invention, small pellets composed of the hydroquinone clathrate of radioactive krypton 85 in a powdered sugar base were placed at location 75 in grease reservoir 70 and at location 74 in channel 71. The drill was then used to drill an exploratory hole. In one test the drill lasted for 7 hours after initial radioactivity was detected at the surface. In another test the drill functioned for hours after the first sign of radioactivity at the surface. The tracer, then, provided drilling personnel with an indication of impending bearing failure. in another embodiment, small pellets of clathrates and kryptonates of krypton 85 are placed at location 85 underneath set screw 85 in the surface of cone 89 and at location 88 under a tooth implanted in the surface of cone 89. The diffusion of the radioactivity in these pellets throughout the dirlling mud as it flows to the surface, coupled with efficient detection of the radioactivity at the surface, gives the drilling crew a clear indication of excessive wear or serious damage to the teeth or cone downhole.

In drilling wellbores, it is well known that the external surface of the drill bit, i.e., the smooth, tapered surface of shank 82 denoted as surface 97 in FIG. 2 and commonly called the shirttail, is subject to wear due to friction with the sides of the previously drilled section of the wellbore. When this occurs bearing seals 80 become exposed and bearing failure is imminent. Also, cone 89 is then subject to shock from the side of the wellbore and will likely fail. Thus, in one embodiment of my invention, a small amount of radioactive krypton 85 in clathrate or water-soluble kryptonate form is placed in shirttail 97 at location 94 beneath set screw 96. If radioactivity is released from this source the drillers will know that drill bit failure is imminent.

While certain specific embodiments have been described herein and depicted in the drawings my invention is intended to be limited only by the scope and spirit of the appended claims.

I claim:

1. in a process for detecting wear and failure of a drill bit during a drilling operation in a borehole in which drilling fluid is continuously recirculated, and wherein a radioactive tracer is placed within a drill bit so that when drill bit wear or failure occurs such failure can be detected by measuring the radioactivity of the recirculating drilling fluid carrying the released radioactive tracer, with said radioactive tracer being detected by beta and gamma ray counting means for indicating both the transient level of radioactivity in said recirculating drilling fluid and the background level of radioactivity in said recirculating drilling fluid as a function of time, the improvement comprising placing krypton contained in a soluble solid form in said drill bit so that upon wear or failure said krypton 85 solid is dissolved in said drilling fluid, passing the recirculating drilling mud after it flows from the borehole through gas-separating means to remove a substantial portion of said krypton 85 as a gas from said drilling fluid, conducting said separated gas from said recirculating drilling fluid to said beta and gamma ray counting means, and detecting an abrupt rise in the transient level of radioactivity due to the presence of beta rays in said gas as a primary indication of wear or failure, with the relatively low level of background radioactivity in the recirculated drilling fluid being indicated by beta and gamma rays from gases continuously extracted from said recirculated drilling fluid.

2. In a process for detecting wear and failure of a drill bit having a plurality of rotatable cutters during a drilling operation in a borehole in which drilling fluid is continuously recirculated, and wherein a radioactive tracer is placed within a drill bit so that when drill bit wear or failure occurs such failure can be detected by measuring the radioactivity of the recirculating drilling fluid carrying the released radioactive tracer, with said radioactive tracer being detected by beta and gamma ray counting means for indicating both the transient level of radioactivity in said recirculating drilling fluid and the background level of radioactivity in said recirculating drilling fluid as a function of time, the improvement comprising placing krypton 85 dissolved in a carrier in a grease reservoir or a sealed bearing for at least one of said plurality of rotatable cutters of said drill bit so that upon wear or failure of said bearing said krypton 85 is dissolved in said drilling fluid, passing the recirculating drilling mud after it flows from the borehole through gas-separating means to remove a substantial portion of said krypton 85 as a gas from said drilling fluid, conducting said separated gas from said recirculating drilling fluid to said beta and gamma ray counting means, and detecting an abrupt rise in the transient level of radioactivity due to the presence of beta rays in said gas as a primary indication of bearing wear or failure, with the relatively low level of background radioactivity in the recirculated drilling fluid being indicated by gamma rays from gases continuously extracted from said recirculated drilling fluid.

3. The method of claim 2 wherein said carrier is grease and the radioactive krypton is physically dissolved in a portion of the grease in said reservoir. 

2. In a process for detecting wear and failure of a drill bit having a plurality of rotatable cutters during a drilling operation in a borehole in which drilling fluid is continuously recirculated, and wherein a radioactive tracer is placed within a drill bit so that when drill bit wear or failure occurs such failure can be detected by measuring the radioactivity of the recirculating drilling fluid carrying the released radioactive tracer, with said radioactive tracer being detected by beta and gamma ray counting means for indicating both the transient level of radioactivity in said recirculating drilling fluid and the background level of radioactivity in said recirculating drilling fluid as a function of time, the improvement comprising placing krypton 85 dissolved in a carrier in a grease reservoir or a sealed bearing for at least one of said plurality of rotatable cutters of said drill bit so that upon wear or failure of said bearing said krypton 85 is dissolved in said drilling fluid, passing the recirculating drilling mud after it flows from the borehole through gas-separating means to remove a substantial portion of said krypton 85 as a gas from said drilling fluid, conducting said separated gas from said recirculating drilling fluid to said beta and gamma ray counting means, and detecting an abrupt rise in the transient level of radioactivity due to the presence of beta rays in said gas as a primary indication of bearing wear or failure, with the relatively low level of background radioactivity in the recirculated drilling fluid being indicated by gamma rays from gases continuously extracted from said recirculated drilling fluid.
 3. The method of claim 2 wherein said carrier is grease and the radioactive krypton is physically dissolved in a portion of the grease in said reservoir. 